I want to Post a string which is returned from a plugin as Output Parameter as part of the text in a post of a opportunity timeline.

What i tried:
I created an action and registered (PostOperation, synchronous) the plugin to that message name. The action recieves an input parameter (ooportunity reference) and has an output Parameter "PostMessage". When i trigger the action by using CRM REST Builder i can see that the Output-Parameter "PostMessage" is returned.

To create the post which states the Ouput Parameter i tried two different approaches. As you can see in the screenshot i create a step "create post" and set the postMessage-Argument as variable in the text. The post is created but the variable seems to be emtpy. The same result with a separate action which is called by the first one (PostMessage and Opportunity also passed to that action)

What am i doing wrong?